1. Field of the Invention
The present invention relates to programmable integrated circuits. More particularly, the present invention relates to a programmable logic module and architecture for use in integrated circuits such as field programmable gate array integrated circuits.
2. The Prior Art
Recent advances in user-programmable interconnect technology have resulted in the development of field programmable gate array (FPGA) integrated circuits which may be customized by a user to perform a wide variety of combinational and sequential logic functions. Numerous architectures for such integrated circuits are known. Examples of such architectures are found disclosed in U.S. Pat. No. 4,870,302 to Freeman, U.S. Pat. No. 4,758,745 to El Gamal et al., and U.S. Pat. No. 5,132,571 to McCollum et al. The architecture employed in a particular FPGA integrated circuit will determine the richness and the density of the possible interconnections which can be made among the various circuit elements disposed on the integrated circuit and thus can profoundly affect its usefulness.
Programmable logic blocks which are capable of performing a selectable one of a plurality of user-selectable logic functions are known in the prior art and are employed in FPGA architectures such as the ones cited above. U.S. Pat. No. 4,910,417 to El Gamal et al., assigned to the same assignee as the present invention, U.S. Pat. Nos. 5,055,718 and 5,198,705 to Galbraith et al., and U.S. Pat. No. 4,453,096 to Le Can et al., disclose logic modules composed of multiplexers capable of performing a plurality of combinatorial functions. U.S. Pat. No. 4,541,067 to Whittaker discloses how to perform different logic functions using different combinations of pass transistors.
While these circuits provide a degree of flexibility to the designer of user-programmable logic arrays, there is always a need for improvement of functionality of such circuits.